[Ingest] Support removing a data source

[jen-huang]

Removing a data source should check if any policies are using that data source, and unassign the data source from those policies first. (This will trigger a policy update.)

In the UI, it would be nice if we are also able to warn the user how many policies will be affected by removal of a data source.

[elasticmachine]

Pinging @elastic/ingest (Feature:EPM)

[ruflin]

There is also the data that belongs to a data sources through its input. To simplify things, I would suggest as long as a data source is still assigned to a policy, it can't be removed. It must be done manually.

[jen-huang]

As the model for datasources has changed, the concerns around a deleted datasource's related objects aren't a problem any more. Deletion API was added in #59848 and UI was added in #60429. Closing this issue.